Rope terminators are sometimes referred to as “Rope Fittings” and are attached to an end of a rope to provide a means for securing the rope end to an anchor point. The rope terminator of the present invention is particularly intended for use with parallel core ropes formed from high strength, low stretch synthetic fibres for example Aramid fibres, a particular example being Kevlar™, and those consisting of parallel PBO (p-phenylylene-2′6-benzobisoxazole) fibres, an example of which is Zylon™.
The term “parallel core” in relation to ropes indicates that the fibres of the rope extend parallel to one another as a “hank” of fibres usually restrained between their ends by an outer sheath or binding. However in so far as the terminator is concerned the fibres need to be parallel only in the immediate proximity of the terminator and so it is to be understood that ropes where the fibres are wound or even braided can be used with the terminator provided that the fibres at the terminator end region of the rope are arranged generally parallel to one another. Moreover while a terminator in accordance with the present invention is intended for use with Aramid or PBO fibre ropes, it can be used with ropes formed from fibres of other materials.
A conventional rope termination is illustrated in British Patent 1341013 and in FIG. 1 of the accompanying drawings. A tapering wedge member or spike 3 traps fibres of the rope 9 against the inner surface of an internally tapering metal body 1. As the tension in the rope increases the wedging action between the wedge member, the fibres, and the barrel increases as the wedge member is drawn deeper into the tapering barrel. The arrangement illustrated in FIG. 1 and British Patent 1341013 is suitable for terminating steel strand ropes, natural fibre ropes, and ropes constructed from certain synthetic fibres.
High strength, low stretch fibres are used for ropes in applications where high strength and/or low stretch combined with low weight is critical. They are typically used in applications where steel rod or steel wire would conventionally be used but are deemed too heavy, including by way of example any high performance sporting equipment where weight reduction is generally of prime concern, of which yacht rigging is a prime example. Although rope terminators in accordance with the present invention can find use in a wide variety of applications, consideration of the yachting industry will explain the trend towards increased use of modern high strength, low stretch fibre rope materials such as Aramid and PBO materials. Over the past few years the use of steel for deck hardware (pulleys, winches, furling systems and the like) has been replaced by composite materials, such as carbon fibre reinforced plastics materials. Masts and spars, conventionally formed from aluminium, are now increasingly formed from carbon reinforced synthetic resin materials.
The yacht designer's aim is to reduce not simply the overall weight of the yacht, but also to reduce the proportion of the weight of the yacht which is disposed above the yacht centre of gravity. As a simplified example, consider a 20 metre yacht with a 25 metre mast rigged for racing will probably use steel strand ropes or steel rod for its standing rigging, the ropes being terminated with terminators of the kind shown in British Patent 1341013 also formed from steel. Such standing rigging will have a weight of approximately 1.4 kg/m. Approximately 100 metres of the such rigging will be required and if the steel standing rigging could be replaced with a PBO rope of the same strength then the weight of the standing rigging would be reduced from approximately 1.4 kg/m to less than 0.2 kg/m. There would thus be a weight saving of over 100 kg and in the case of standing rigging then the weight of the standing rigging can be considered to be nominally located half way up the height of the mast (12.5 m) and at this height the turning moment of a 100 kg weight is very significant in the handling of the yacht. Furthermore, any reduction in weight in the standing rigging allows approximately a five fold reduction in the counter balancing keel weight and thus it can be seen that by replacing steel standing rigging with PBO fibre rigging an overall weight reduction of 600 kg could be achieved. The PBO ropes will have a cross-sectional area approaching half that of an equivalent modulus steel rope/rod, and consequently the aerodynamic drag or windage presented by PBO ropes will be the same. However, to achieve the same modulus as steel rod, the PBO rope will use more fibre than would be required for strength parity, therefore the PBO rope will have a Nominal Breaking Load (NBL) approximately 40% higher than its equivalent modulus steel rod comparison.
In addition to the performance increase attributable to simply reducing the overall weight of the yacht there would be performance and safety improvements in relation to the responsiveness of the yacht given the lower centre of gravity, reduced draft of the hull, and “tighter” rig. As PBO ropes have the same elastic modulus (and therefore the same stretch) as steel, combined with a reduction in weight of 85% compared to steel, they would form excellent replacements to steel ropes/steel rods in standing rigging where minimising lateral mast movement is critical. However this is not generally possible as the use of a conventional terminator of the kind shown in FIG. 1 and British Patent 1341013 is highly problematic in relation to PBO fibre ropes and consequently PBO is generally not used in conventional standing rigging. The present invention, configured for PBO ropes, permits PBO based ropes to be used in such situations. Although Aramid ropes can be terminated with conventional terminators of the kind shown in FIG. 1 and British Patent 1341013 Aramid ropes are not generally used in shrouds and lateral mast support as they have a lower elastic modulus (and therefore higher stretch) than steel which would induce performance handicaps and instability in the rig. However Aramid ropes are used in the fore and aft plane of the yacht (for example as checkstays and running backstays) where stretch is not as critical but a reduction in weight aloft remains desirable. In these instances where reduction in weight is the prime driver, use of the present invention would offer further weight savings of up to 50% in terminator/fixture weight. Or if PBO cored ropes are used in place of Aramid ropes, an overall weight saving of 50% of assembled rope or ‘stay’ and a reduction in the ‘windage’ would be obtained.
As mentioned above the invention can find use in a variety of different environments where Aramid and/or PBO fibre ropes are or could be used and the invention may enable such ropes to be utilized in applications where their use is desirable but currently difficult or impossible owing to the lack of satisfactory terminations. A non-exhaustive list of possible applications for the invention includes Bridge stays, Antennae supports, Lightweight and temporary building structures, Glass walls/ceilings, Suspended electrical systems, Overhead electrically driven tram & train systems, Offshore mooring systems for example Floating Production, Storage & Offloading (FPSO) vessels, and Mobile Drilling Units (MDU).
It is found that both Aramid, and even more so, PBO fibres exhibit their optimum tensile strength while the fibres are maintained as close to a straight configuration as possible. It has been recognised by the present Inventor that fibres have a Fibre Failure Angle (FFA) when under tensile stress, beyond which the fibres should not be bent if optimum tensile strength characteristics of the fibres are to be maintained. It will be recognised that fibres within a terminator bend in a “soft” manner at the point at which they exit the sheath of the rope (the sheath can expand as the fibres are put in tension so that the bend is spread over a radiused finite distance) and again in a “hard” fixed manner at the point where they begin to be gripped between the wedge member and the barrel of the terminator. Both of these bend angles are largely determined by the internal cone angle of the barrel of the terminator. Fibres such as Aramid and PBO have a relatively small FFA and so the cone angle of the barrel of the terminator should be correspondingly small. However, there is a minimum length of fibre which must be gripped between the barrel and the wedge member to prevent terminator failure through fibre slippage and this length increases as the cone angle of the barrel decreases. This, coupled with increasing rope diameters, for larger load capacities, means that the terminator barrel becomes quite large, and when formed in steel carries with it a significant weight penalty.
Furthermore, the inner surface of the barrel is required to have a high surface finish so as to exhibit a lower frictional grip against the fibres than that exhibited between the fibres and the wedge member. As the size of the barrel increases and the cone angle decreases, the difficulty of producing an appropriate surface finish on the interior of the barrel also increases to the point at which it is at best economically non-viable, and at worst impossible, to produce a satisfactory steel barrel for a PBO rope terminator.
It is an object of the present invention to provide a rope terminator which is suitable, in particular, for terminating Aramid and PBO fibre ropes.